Conceptual Design of a Multitubular Fixed-Bed Reactor for Methanol Ammoxidation to Hydrogen Cyanide over FeMo/SiO2 Using a Mars–van Krevelen Kinetic Model

Methanol ammoxidation over FeMo/SiO2 has emerged as a promising low-temperature route to hydrogen cyanide (HCN). In this work, an eight-parameter Mars–van Krevelen (MvK) kinetic model, previously established from intrinsic fixed-bed experiments, is embedded in a heterogeneous plug-flow description to design an industrial multitubular reactor with a nominal HCN capacity of 10,000 t∙a−1. The reactor is represented by a bank of isothermal tubes that are operated at 420 °C and a mildly elevated pressure, each packed with spherical FeMo/SiO2 pellets. Detailed simulations for a 30 mm inner tube diameter and 2 mm pellets, including an Ergun pressure drop and intraparticle diffusion with realistic effective diffusivities, show that a 4 m bed at an outlet pressure of 1.5 bar (abs) achieves an essentially complete methanol conversion with a carbon-based HCN yield of ≈0.95 at a space time of ≈160 gcat∙h∙mol−1. Axial effectiveness factors remain above ≈0.6, indicating moderate but manageable diffusion limitations. Comparison with a 35 mm/3 mm geometry reveals a clear trade-off between pressure drop and HCN selectivity. Parametric studies of space time, feed composition and outlet pressure delineate a broad non-flammable operating window with robust HCN yield and moderate compression duty. The results demonstrate how a mechanistic MvK rate expression can be translated into a practical design framework for FeMo-based multitubular HCN reactors.